Material development using reproducible resources such as plant raw materials is a research theme of high social interests which should be started urgently from the viewpoint of recycling style social construction. If plastics made of petroleum as main raw materials can be replaced with bioplastics produced from reproducible raw materials, carbon neutrality, i.e. an idea that an overall carbon dioxide content in the air does not increase even when materials incorporated with carbon dioxide are combusted in the environment during the process of photosynthesis, is realized, so that it can be contributed to the prevention of global warming.
Among the bioplastics, polylactic acids which can be produced from reproducible resources such as maize have been remarked with high interests, and the utilization of such polylactic acids has been already started in the medical fields. For the purpose of further application developments, in 2005 Aichi Expo, high potentials of the polylactic acids have been confirmed as practical materials by carrying out large-scaled verification tests and the like, and the polylactic acids have been expected to be utilized in agricultural materials (for example, sheets, films), food wrapping materials (for example, food wrapping films, sheets, bags), and other wrapping materials (for example, clothes, sundries wrapping sheets, films, bags), and the like.
Polylactic acids are crystalline thermoplastic polymers having tensile strength and transparency of the same level as PET (polyethylene terephthalate). The burned calories when combusted are as small as one-third that of PE (polyester), PP (polypropylene) or the like, so that little damage takes place in the combustion furnace or little harmful gases are generated. In addition, since the raw materials for the polylactic acid are plants as mentioned above, an increase in carbon dioxide when subjected to combustion treatment is less likely to cause a load to the environment, so that such materials are also eco-friendly. Because of the advantages mentioned above, research and development on production methods or applied uses have become more actively performed in the recent years, and diversified applications and increases in amounts produced will be expected in future.
However, polylactic acids are rigid and brittle, in other words have the properties with worsened impact properties, so that their industrial applications have been limited. Therefore, various studies have been made in order to improve the physical properties of the polylactic acids.
For example, Patent Publication 1 discloses a resin composition containing a polylactic acid and a flexible biodegradable aliphatic polyester such as polycaprolactone, polybutylene succinate or polyglycolide, wherein the polylactic acid is mixed with the flexible biodegradable aliphatic polyester.
Also, Patent Publication 2 discloses a polylactic acid-based resin composition added with a plasticizer such as di-i-butyl adipate or di-n-butyl sebacate, and Patent Publication 3 discloses an aliphatic polyester composition in which an aliphatic polyester, especially a polylactic acid, is blended with a polyglycerol acetic acid ester as a plasticizer.
Further, Patent Publications 4 and 5 disclose particular polymers comprising two polymers each satisfying particular relationships derived from dissolubility parameter values and density values as impact resistance enhancing agents for polylactic acids.
Furthermore, Patent Publication 6, an invention of a patent application previously filed by the inventors of the present application, reports that plasticity of a polylactic acid can be improved by adding to a polylactic acid a polyglycerol fatty acid ester having particular average degree of polymerization and fatty acid esterification ratio.